Our first major objective is to determine whether or not rats that exercise regularly live longer than freely eating sedentary controls, or than paired weight sedentary controls that are food restricted to maintain their body weights the same as those of the exercisers. Our second major objective is to determine whether exercise slows, or compensates for, certain of the deleterious changes in structure and function that occur with aging. We propose to evaluate the effects of exercise on changes with age in: 1. the respiratory capacity of heart and skeletal muscle; 2. the size, number and structure of skeletal muscle fibers; 3. the O2 uptake capacity, fatigue resistance, and high energy phosphage stores of skeletal muscle; 4. the contractile properties of fast-twitch and slow-twitch skeletal muscles; 5. the prolongation of the time to peak tension and the half relaxation time of the myocardium; 6. the rate of protein synthesis in the cerebral cortex; 7. thyroid hormone turnover; 8. whole body minimal O2 composition; 9. obesity, lean body mass, and bone; 10. glucose tolerance and insulin sensitivity; and 11. the sensitivity of the pancreas (beta-cells) to stimulation of insulin release by glucose and arginine, and on the maximum capacity to secrete insulin. Our third objective is to further test the validity of the "rate of living" theory of aging by determining the effect on longevity of elevation of metabolic rate in rats by means of cold exposure.